Add missing Clone trait to code

[overcat]

Clone trait is required, without it the code cannot be compiled.

error[E0277]: the trait bound `RawVal: From<&Name>` is not satisfied
  --> src/lib.rs:23:1
   |
23 | #[contractimpl(export_if = "export")]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `From<&Name>` is not implemented for `RawVal`
   |
   = help: the following other types implement trait `From<T>`:
             <RawVal as From<&()>>
             <RawVal as From<&ScStatus>>
             <RawVal as From<&i32>>
             <RawVal as From<&u32>>
             <RawVal as From<()>>
             <RawVal as From<Account>>
             <RawVal as From<FixedBinary<N>>>
             <RawVal as From<ScStatus>>
           and 19 others
   = note: required because of the requirements on the impl of `Into<RawVal>` for `&Name`
   = note: required because of the requirements on the impl of `IntoVal<Env, RawVal>` for `&Name`
   = note: this error originates in the attribute macro `contractimpl` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0277`.
error: could not compile `custom-type` due to previous error

I think this might be confusing for beginers, but if this is intentional, please feel free to close this PR